The invention relates to a method for correcting the frictional losses during the simulation of masses on stationary test stands or benches, as well as an apparatus for carrying out this method.
For carrying out stationary motor vehicle testing, primarily roller test benches are used as stationary test benches. In this context, the drive wheels of a motor vehicle are coupled with the rollers of the test bench with regard to rotational moments. Since the motor vehicles do not move on these test benches, the inertial forces resulting from the acceleration of the motor vehicles mass must be simulated by the test bench, if the mass inertial moment of the test bench does not correspond with that of the motor vehicle mass. In this context it is typical to simulate the difference of the inertial forces through a loading moment, which is, for example, generated by means of a direct current machine. The magnitude of the loading moment is regulated by a dynamic regulation loop or circuit dependent on the respective acceleration.
For example, in order to exactly determine the exhaust gas properties of a motor vehicle determined on such test benches, the automobile industry and the environmental agencies place high demands on the measuring accuracy of such roller test benches. Therefore, the above mentioned loading moment may not be falsified by measuring errors relating to or arising from the test bench. Thus, previously, a great focus of attention has been placed on the selection and the positioning of the test bench bearings, so that the measuring result will not be falsified by bearing friction, to the extent possible. For this reason, it has been a known practice to select bearings having friction values that are as small as possible and are reproducible, which are then taken into account in the measuring result. Thus, it is also a known practice to record a velocity dependent loss characteristic function of the test bench in known time intervals, and to calculate these losses into the rated or nominal values of the load. Thereby, it was possible to achieve and maintain the test bench related measuring errors of  less than xc2x10.1% of the force nominal value, as required by the U.S. Environmental Protection Agency (EPA).
Nonetheless, these high measuring accuracies can only be maintained if the test bench losses do not vary after recording or taking up the loss characteristic function, or if a new loss characteristic function is taken up or recorded after each variation and is then also taken into account in the simulation. Therefore, such calibrations have been renewed or repeated at certain known time intervals, or upon a change of corresponding influence factors, in order to sufficiently take into account longer term variations of the influence factors. In order to detect all variations over time to the extent possible, the loss characteristic function would basically have to be carried out before each new testing process, which is not practical due to the great effect and expense. Even with such efforts, the changes that arise during the time progression of the test process would still not be detectable, and would then falsify the measuring result.
It is therefore the object of the invention to provide a method and an apparatus that will take into account as many test stand losses as possible, and especially losses related to a series of time varying changes arising during the testing operation.
The above object has been achieved according to the invention in a method for taking into account frictional losses of a stationary test stand in a simulation of masses on the stationary test stand, in which velocity dependent frictional losses for taring the test stand are determined with the aid of a loss power recording dependent on the velocity at which the test stand is operating. The inventive improvement comprises determining at least one of air frictional losses of the test stand and load dependent bearing frictional losses of the test stand with the aid of the loss power recording together with at least one of an actual existing air characteristic value and an actual existing bearing loading, and correcting a loading moment developed by the test stand on the basis of the air frictional losses or the load dependent bearing frictional losses.
The above object has further been achieved according to the invention in a method of operating a stationary test stand including rotating test stand components for testing a running operation of a motor vehicle on the test stand, comprising a preliminary step of determining a friction loss function that defines a total friction loss of the rotating test stand components, wherein the friction loss function includes at least one of a mechanical friction loss term that is dependent on an operating velocity of the rotating test stand components and a load weight applied to bearings of the rotating test stand components, and an aerodynamic friction loss term that is dependent on the operating velocity and on at least one ambient air characteristic parameter of ambient air surrounding the rotating test stand components, wherein the preliminary step of determining the friction loss function comprises carrying out a calibration run of operating the rotating test stand components at plural velocities, and for each respective velocity of the plural velocities determining and recording a value of the total friction loss of the rotating test stand components at the respective velocity, and recording at least one of the load weight applied to the bearings and the at least one ambient air characteristic parameter pertaining during the calibration run.
The inventive method then further involves testing a motor vehicle on the test stand by positioning the motor vehicle on the test stand by positioning the motor vehicle in drive-transmitting engagement with the test stand components, rotating the test stand components at an operating velocity, measuring or specifying at least one of an operating value of the load weight applied to the bearings based on a weight of the motor vehicle and an operating value of the ambient air characteristic parameter, determining an operating value of the total friction loss of the rotating test stand components by evaluating the friction loss function in connection with at least one of the operating values of the load weight and the operating value of the ambient air characteristic parameter, and exerting a loading moment on the rotating test stand components while correcting the loading moment to compensate for the operating value of the total friction loss of the rotating test stand components.
Also, the invention achieves the above object in a stationary test stand for simulating masses, including an apparatus for taking into account frictional losses on the test stand, comprising a correction circuit that has inputs for at least one actual existing air characteristic value and/or an actual existing bearing loading value, and that calculates a respective air frictional loss of the test stand and/or a respective load dependent frictional loss of the test stand, from the at least one actual existing air characteristic value and/or the actual existing bearing loading value, and that corrects a developed loading moment on the basis of the air frictional loss and/or the load dependent frictional loss.